The present invention relates to chromatography systems used for chemical analysis and, more particularly, to an improved septum for an injection port of a gas chromatography system.
In gas chromatography, samples are separated into their components by passing the sample through a separating column. The sample is introduced into a flowing carrier gas in the inlet system. The carrier gas sweeps the sample from the inlet system onto the separating column. The separated components emerging from the column are eluted through a detector which monitors the elution over time. A chromatogram, representing eluting quantity as a function of time, is generated by plotting the detector output signal.
Typically, the carrier gas is sealed from the outside world by a rubber septum which can be cut from a sheet of rubber approximately 1/8 inch thick. The sample is introduced into the inlet system through the septum using a syringe. The sample is drawn into the syringe, then the syringe needle is used to pierce the septum; the sample is injected into the inlet system and the syringe needle is withdrawn.
The septum must serve two functions. First, it must form a seal around the needle to prevent leaks while the injection is effected. Second, it must reseal the injection port after the syringe needle has been withdrawn and maintain this seal while the chromatographic separation is being executed. A leak occurring while the needle is in the septum can cause part of the injected sample to escape the injection system. This "injection" leak is difficult to detect because it occurs during the dynamic process of injection and is only apparent by careful examination of the quantitative results of the chromatogram. A failure to seal after the needle is withdrawn is more easily detected. The resulting "post-injection" leak is evident from variations in the characteristic retention time for a chromatographic peak resulting from variations in the column flow rate. Some capillary column injection ports may show different behavior for a post-injection leak because of their flow configuration.
Septum deterioration and resulting failure are serious problems. Monitoring a system for leaks can be costly and time consuming. Replacement of seals is incovenient. Problems of detection and replacement are aggravated in automated systems which may run unattended for as many as 100 samples. A failure early in a run can impair the validity of the results for all subsequent samples.
Septum deterioration is inevitable due to the action of the syringe needles used for injection. Syringe needles must be strong enough to pierce the septa of sample containers and injection ports without bending or buckling. This strength requirement leads to the use of larger diameter needles. Larger diameter needles require greater insertion force to pierce a septum, which is thus subject to greater wear. The larger needles also make larger holes or tears in the septum, which are harder to reseal after needle withdrawal. Needles of smaller diameter cause less damage to the septum and make resealing easier, but are much more susceptible to bending when piercing a seal.
Syringe needles are made with sharp, beveled points to slice through the septa with lower force. However, the slicing action of repeated injections with beveled needles can macerate a septum, which then begins to leak. In addition, small pieces of rubber torn from a septum by the needles can fall into the injection port liner. Once in the liner, these pieces can affect the analysis in two ways. First, they can release compounds which can appear as "ghost peaks" in the chromatogram. Second, they can adsorb or partition sample components as they pass through the injection port, causing distortion of peak shapes in the chromatogram. In either case, the validity of the resulting chromatogram is impaired.
Taking into account these considerations, most gas chromatographs use a rubber septum, approximately 3 mm thick and 6-12 mm in diameter. Syringes typically used with such septa are 10 microliter total capacity, with a sharp beveled 26 gauge, i.e., 0.48 mm diameter, needle. These are used for both manual injection and for automatic liquid samplers. Alternatively, thicker "cylindrical" septa are used to improve the reliability of sealing after multiple injections. These have the disadvantage of requiring higher syringe force.
Especially strenuous demands are made on a septum in automatic liquid samplers, such as the Hewlett-Packard 7673A. This sampler uses a very rapid injection cycle. A differently shaped needle is adapted so that it can pierce a septum so that liquid sample can be injected into the injection port liner. The needle can be removed from the port in less than 0.25 seconds before the syringe needle contents can be heated significantly by the injection port or the septum. The needle has a relatively large diameter, e.g., 0.66 mm, to allow it to withstand the higher force required to pierce the septum at high speed without bending or buckling. The needle has a blunt tip which facilitates a properly directed spray pattern for the sample. The larger diameter and blunt tip cause greater damage to the septum per injection, thus shortening the septum life before leaks occur or pieces of septum fall into the injection liner.
The problem of maceration can be addressed by using a septum having a predefined path for needle penetration. For example, septa have been adapted from "duckbill" seals. A duckbill seal comprises a flat rubber tube with two flat surfaces which can seal against each other. Duckbill seals are often used as check valves in flow systems because they open with very low pressure drop in one direction while sealing in the other direction. Duckbill seals are effective under high pressure, which causes the flat surfaces to press against each other more tightly.
Because a slit is preformed between the flat surfaces, a blunt needle can be inserted with low force through a duckbill seal many times without tearing or crumbling the rubber by forcing the flat surfaces apart. However, the flat seals do not form an effective seal about a syringe needle so leaks can occur during injection. In addition, the low pressure drops across the seal can be insufficient to close the flat surfaces, allowing post-injection leaks. The problem with post-injection leaks can be remedied by adding a spring to force the sealing surfaces together. Another disadvantage of duckbill seals is that they have a long aspect ratio and do not fit into a conventional septum holder and therefore require a specially designed seal holder.
A duckbill seal has been included in an inlet assembly for a capillary column. For example, in the "Hewlett-Packard 1988 Analytical Supplies Catalog and Chromatography Reference Guide", page 35, a Cool On-Column Inlet is described including a duck bill. Inspection of the actual system reveals that a stainless steel probe is used to separate the duckbill surfaces. Once the surfaces are separated by the probe, the needle is extended through the probe and the probe is withdrawn so that the duckbill closes around the needle. Neither the probe nor the duckbill seals the needle completely, so that some leakage generally occurs. It is noted that the inner perimeter of the duckbill seal is about 1.6 mm at the duckbill end and about 3.1 mm at the end where the needle is first inserted, so that this latter end never forms a seal with the needle.
The foregoing are but a few of the wide variety of commercially available septa. Other examples are described in catalogues from gas chromatography suppliers. The wide variety of septa available with differing claims for pierceability, temperature stability, and number of injections show that there is a need for a more durable septum. In addition, the availability of needle guides and a needle guide with a "backup" seal in case the septum fails is further evidence that there is a need for an improved and more reliable septum. Preferably, such a septum would accommodate conventional form factors rather than requiring a specially designed seal holder.